These behemoths — which lie at the hearts of most if not all galaxies, and can contain billions of times more mass than the sun — probably form after the collapse of gigantic gas clouds, rather than the death of individual stars, as some astronomers had thought, the study suggests.

"There is a lot of controversy over which path these black holes take," study co-author Andrea Ferrara, of Scuola Normale Superiore in Pisa, Italy, said in a statement. "Our work suggests we are converging on one answer, where black holes start big and grow at the normal rate, rather than starting small and growing at a very fast rate." [Images: Black Holes of the Universe]

"Black hole seeds are extremely hard to find, and confirming their detection is very difficult," co-author Andrea Grazian, from the National Institute for Astrophysics in Italy, said in the same statement. "However, we think our research has uncovered the two best candidates so far."

Both of the seed candidates contain about 100,000 solar masses, and formed less than 1 billion years after the Big Bang (which created the universe about 13.8 billion years ago). The seeds' size and age imply that, at least in the early universe, supermassive black holes started out large, with the collapse of giant gas clouds, the researchers said.

Smaller black holes do exist, of course; the gravitational collapse of huge, dead stars gives rise to stellar-mass black holes, which contain 10 to 100 times the mass of the sun. And these objects can get bigger, by merging with other black holes and sucking up nearby gas. But such small seeds likely do not grow into the monsters at the hearts of galaxies, the new study suggests.

The researchers stressed that their conclusions are preliminary. For example, more observations are needed to confirm that the two candidates are indeed supermassive black hole seeds, as suspected. And drawing firm conclusions about such seeds' origins will require finding and studying more than two of the objects, team members said.

The new study has been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.